(1) Field of the Invention
This invention relates to a communication system and, more particularly, to a communication system for performing telecommunication on an asynchronous network.
(2) Description of the Related Art
With an explosive increase in traffic on IP networks the nucleus of which is the Internet, in recent years image/speech data has been delivered extensively. It is hoped that more advanced services will be provided in a wider area.
To transmit image/speech data in real time, sending timing must match receiving timing. With asynchronous IP networks, however, a sending end and a receiving end do not have a common reference clock. Accordingly, a receiver must generate data which is in synchronization with a clock at the sending end. Processes, such as decoding, are performed on this data.
FIG. 10 shows the configuration of a communication system using a conventional receiver. A communication system 1a is used for performing constant bit rate (CBR) communication in which a certain band of traffic is secured by using a fixed transmission rate. A transmitter 100 is connected to an IP network 31. The IP network 31 is connected to a LAN 32. The LAN 32 is connected to a receiver 200.
The receiver 200 includes an I/F section 206, a memory 202, and a phase locked loop (PLL) control section 204. The PLL control section 204 includes a stored data amount monitoring section 204a and a voltage controlled oscillator (VCO) 204b. 
When the I/F section 206 receives send data sent from the transmitter 100 via the IP network 31 and the LAN 32, the I/F section 206 extracts effective data from the send data and sends it to the memory 202. The I/F section 206 also outputs a transmission clock on the LAN 32 connected thereto as a write clock WCK used for writing data to the memory 202.
To be concrete, the I/F section 206 corresponds to a network interface card (NIC), being a expansion card used for connecting a personal computer, a printer, or the like to a LAN. If the LAN 32 is, for example, 10 megabit Ethernet (registered trademark), then the rate of the write clock WCK is 10 Mb/s.
The effective data is written to the memory 202 by using the write clock WCK and is read out from the memory 202 by using a read clock RCK. The stored data amount monitoring section 204a monitors the differential between the amount of data stored in the memory 202 and a set threshold, and outputs differential information.
The VCO 204b changes its oscillation frequency on the basis of the differential information so that the differential between the amount of data stored in the memory 202 and the set threshold will become zero, and generates and outputs the read clock RCK.
That is to say, if the VCO 204b recognizes on the basis of the differential information that the amount of data stored in the memory 202 is smaller than the set threshold, then the VCO 204b makes the read clock RCK slow and reads out the data from the memory 202. On the other hand, if the VCO 204b recognizes on the basis of the differential information that the amount of data stored in the memory 202 is greater than the set threshold, then the VCO 204b makes the read clock RCK fast and reads out the data from the memory 202. By doing so, the VCO 204b keeps the amount of data stored in the memory 202 constant (that is to say, the VCO 204b keeps the read clock RCK constant)
As stated above, data is sent at a constant rate, and control is exercised over reading so that the amount of data stored in the memory 202 included in the receiver 200 will be constant (however, the rate of the write clock WCK is slower than or equal to the rate of the read clock RCK). As a result, read data can be generated in synchronization with the send rate.
A technique in which, at a sending end, time information regarding an asynchronous clock is counted with a clock used on a synchronous network as reference and a count is added to a packet as a time stamp and in which, at a receiving end, reproduction timing is generated on the basis of the time stamp with the clock used on the synchronous network as reference has conventionally been proposed as a technique for re-creating reproduction timing at the receiving end (see, for example, Japanese Patent No. 2861515, paragraphs [0024]-[0026] and FIG. 1).
The receiver 200 shown in FIG. 10 is applicable to CBR communication in which a bit rate is fixed. With MPEG compression of a picture, variable bit rate (VBR) communication in which a large number of bits are assigned to a portion where a pattern is complex and in which a small number of bits are assigned to a portion where a pattern is simple is used. The receiver 200 shown in FIG. 10 is not applicable to such VBR communication.
It is assumed that a transmission rate is 6 Mb/s. With the CBR communication, the number of bits is 6 Mb at any time, so the amount of data stored in the memory 202 increases at a constant rate. Accordingly, to keep the amount of data stored in the memory 202 constant, a threshold is set, PLL control is exercised, and reading is performed at a constant read rate. This is the conventional way. By doing so, data can be read out from the memory 202 at the intervals at which data is generated at a sending end.
On the other hand, with the VBR communication the number of bits differs among different portions (if a transmission rate is 6 Mb/s, then the total number of bits in some time zone is 6 Mb), so the amount of data stored in the memory 202 does not increase at a constant rate. If the conventional PLL control in which the amount of data stored in the memory 202 is kept constant by using a fixed threshold is exercised in such a state, then a read clock synchronized with a clock at a sending end cannot be reproduced and therefore data cannot be read out accurately from the memory 202 at the intervals at which data is generated at the sending end. This causes the problem of, for example, a color shear in an image or speech slowness/fast forward.
Moreover, if a packet loss occurred on the IP network 31, the receiver 200 cannot accurately re-create the intervals at which data is generated at the sending end for the same reason that is stated in the case of receiving VBR data. In addition, with the conventional technique (disclosed in Japanese Patent No. 2861515), a reproduction clock is generated at the receiving end on the basis of the clock used on the synchronous network on the condition that packet communication is performed between a transmitter and a receiver which are asynchronous to each other on the network on which synchronization is established by the clock. Accordingly, this technique is not applicable to packet communication on a completely asynchronous network.